A plotline could involve Petunia having a change of heart due to a magical event. For instance, she might accidentally stumble upon some of Harry's magic or receive a message from the wizarding world about his importance. This could lead her to stand up against Vernon and protect Harry. She could then start to teach Harry about his mother's side of the family and the magic that runs in his veins in a more positive way than the Dursleys ever did. And perhaps she even helps him develop his magic skills secretly before he goes to Hogwarts.
One possible plotline could be that Petunia has a sudden realization of how unfairly she has treated Harry over the years and tries to make amends. She might start by secretly helping him with things around the house or protecting him from Dudley's bullying.
One possible plotline could be Petunia struggling to make ends meet while raising Dudley on her own. She might have to take on extra jobs in the Muggle world, and this could lead to some interesting interactions with her neighbors. Another could be her dealing with Dudley's magical abilities (if he has any latent magic), which she has no idea how to handle.
One possible plotline could be that Lockhart attempts to use Harry for some self - promoting stunt that goes horribly wrong, and Severus steps in to rescue Harry from the immediate danger. For example, Lockhart might try to make Harry participate in a dangerous magic display for his own glory, but Severus, seeing the real danger, intervenes.
One possible plotline could be that Voldemort realizes that Harry is the key to some greater power or knowledge that he wants to obtain. So, instead of killing Harry, he saves him from another dark wizard or a dangerous magical creature. Maybe he sees that Harry has a connection to an ancient magic that could help him rule the wizarding world in a more complete way.
It could be that a powerful sorcerer in the Avengers' world goes rogue and starts using dark magic. The Avengers don't know how to handle magic, so Harry is called in. He has to face this rogue sorcerer, using his own brand of light magic. Along the way, he might have some clashes with the Avengers' way of doing things, but in the end, they all work together and Harry manages to save the day by defeating the rogue sorcerer and restoring peace in the Avengers' world.
In some fanfictions, Bellatrix might save Harry because she is magically bound to him in some way. Maybe a spell backfired and now she has to protect him. Or perhaps she has a secret love for Sirius Black (Harry's godfather) and in order to honor his memory, she saves Harry. There could also be a plotline where Bellatrix is tired of Voldemort's tyranny and sees Harry as the one who can overthrow him, so she saves him for the greater good of the wizarding community.
One possible plotline could be that Susan and Amelia Bones are in danger from dark wizards. Harry, using his skills and courage, battles the dark wizards and rescues them. After that, there could be a sub - plot about Susan being grateful and perhaps developing a friendship or a deeper connection with Harry.
One possible plotline could be a love - at - first - sight scenario where Porky Pig is smitten with Petunia and tries to court her in his bumbling but endearing way.
One possible plotline is that Harry is magically transported to the Naruto world and discovers Naruto is in danger from a new, powerful enemy. Harry then uses his magic to fight alongside Naruto and his friends, saving Naruto in the process.
One possible plotline could be that Voldemort discovers Harry's abuse through his Death Eaters' spying. He decides to save Harry as he wants to turn Harry to his side and believes that by rescuing him, Harry will be more likely to join him. Another could be that Voldemort has a moment of self - reflection and realizes that the way Harry is being treated is wrong, so he steps in.